The vertebrate retina receives, processes and transforms visual information. The research program elucidates the relationships between receptor expression on retinal neurons, the neural circuitry of the retina, and retinal information processing tasks. The study currently examines the zebrafish model using, as tissue source, acutely dissociated zebrafish retinal neurons and zebrafish retinal slices. Acutely dissociated retinal bipolar and horizontal and photoreceptor cells are readily identified morphologically in culture. Amacrine and ganglion cells can be identified through labeling techniques. In retinal slices cell structure is delineated by dye diffusing from the recording electrode. Physiological responses to neurotransmitters can be assayed with voltage-sensitive probes in isolated cells, and by whole-cell patch recording in retinal slice. In performing information processing tasks, retinal neurons express unique neurotransmitter responses. Bipolar cells may be either excited or inhibited by glutamate released from photoreceptors forming ON and OFF pathways. GABA-generated inhibitory responses are commonly of the non-desensitizing, bicuculline-insensitive GABA-C type. Further descriptions of retinal neural processing may be found at http://webvision.med.utah.edu/. Whole-cell patch recording and puff pipette techniques were used to identify glutamate receptor mechanisms on bipolar cell (BC) dendrites in the zebrafish retinal slice. Recorded neurons were stained with Lucifer Yellow, to correlate glutamate responses with BC morphology. BC axon terminals (ATs) consisted of swellings or varicosities along the axon, as well as at its end. AT stratification patterns identified three regions in the inner plexiform layer (IPL): a thick sublamina a, with three bands of ATs, a narrow terminal-free zone in the mid-IPL, and a thin sublamina b, with two bands of ATs. BCs occurred with ATs restricted to sublamina a (Group a), sublamina b (Group b) or with ATs in both sublaminae (Group a/b). OFF-BCs belonged to Group a or Group a/b. These cells responded to glutamate or kainate with a CNQX-sensitive conductance increase. Reversal potential (Erev) ranged from -0.6 to +18 mV. Bipolar cells stimulated sequentially with both kainate and glutamate revealed a population of glutamate-insensitive, kainate-sensitive cells in addition to cells sensitive to both agonists. ON-BCs responded to glutamate via one of three mechanisms: (a) a conductance decrease with Erev ~ 0 mV, mimicked by L-(+)-2-amino- 4-phosphonobutyric acid (APB) or trans-1-amino-1,3- cyclopentanedicarboxylic acid (trans-ACPD), (b) a glutamate- gated chloride conductance increase (IGlu-like) characterized by Erev near ECl (where ECl is the chloride equilibrium potential) and by partial blockade by extracellular Li+/Na+ substitution or (c) the activation of both APB and chloride mechanisms simultaneously to produce a response with outward currents at all holding potentials. APB-like responses were found only among BCs in Group b, with a single AT ramifying deep within sublamina b; whereas, cells expressing IGlu-like currents had one or more ATs, and occurred within Groups b or a/b. Multistratified cells (Group a/b) were common and occurred with either ON- or OFF-BC physiology. OFF-BCs typically had one or more ATs in sublamina a and only one AT in sublamina b. In contrast, multistratified ON- BCs had one or more ATs in sublamina b and a single AT ramifying deep in sublamina a. Multistratified ON-BCs expressed the IGlu- like mechanism only. Visual processing in the zebrafish retina involves at least 13 BC types. Some of these BCs have ATs in both the ON- and OFF-sublaminae, suggesting a significant role for ON- and OFF-inputs throughout the IPL.